Natural gas hydrates are crystalline solids composed of water and gas. In these solids, the gas molecules (guests) are trapped in water cavities (host) that are composed of hydrogen-bonded water molecules. Methane is the main gas in naturally occurring gas hydrates, however carbon dioxide, hydrogen sulfide, and less frequently, other hydrocarbons such as ethane and propane can be found within the hydrate structure. In 1934, Hammerschmidt determined that natural gas hydrates were blocking gas transmission lines, frequently at temperatures above the ice point. This discovery caused a more pragmatic interest in gas hydrates and led to the regulation of the water content in natural gas pipelines.
Gas hydrates can be easily formed during the transportation of oil and gas in pipelines under certain conditions. Factors affecting gas hydrate formation include gas composition, water content, temperature and pressure, particularly low temperature and high pressure. While these crystalline cage-like structures are small initially, they are able to agglomerate into solid masses called gas hydrate plugs. The formation of gas hydrates within a pipeline often results in lost oil or gas production, damage to transmission lines and equipment, and safety hazards to field workers.
Three types of hydrate inhibitors are currently available to the energy industry for controlling gas hydrates: thermodynamic hydrate inhibitors (THIs), kinetic hydrate inhibitors (KHIs), and anti-agglomerants (AAs). Thermodynamic inhibitors are substances that can reduce the temperature at which gas hydrates form at a given pressure and water content. Methanol and ethylene glycol are among the most common thermodynamic inhibitors used in the oil industry. However, thermodynamic inhibitors often have to be added in large amounts to be effective, typically in the order of several tens of percent by weight of the water present. Therefore, there is a substantial cost associated with transportation and storage of large quantities of THIs.
A more cost-effective alternative is the use of kinetic hydrate inhibitors and anti-agglomerants, which are known collectively as low-dosage hydrate inhibitors (LDHIs), reflecting the much lower dosage requirements compared with THIs. Typically, KHIs are low molecular weight polymers that adsorb on gas hydrate crystal faces and interfere with the nucleation and growth of gas hydrate crystals. Unfortunately, there are several limitations that have been discovered with the use of KHIs such as subcooling limits, solubility problems based on temperature and salt content of the water, chemical incompatibility with the system being treated, and expense of the polymers used.
Anti-agglomerants are an alternative to THIs and KHIs. Anti-agglomerants are surface active molecules that attach to and disperse fine gas hydrate crystals, preventing their agglomeration and growth into masses that could become plugs. When small gas hydrate crystals begin to form, AAs attach to them to make the surface hydrophobic, which mediates the capillary attraction between the crystals and water and disperses the crystals into the hydrocarbon phase. This results in a transportable slurry that can flow to the processing facility.
AAs are a cost effective alternative to THIs and are not generally sensitive to changes in system subcooling like KHIs. However, since AAs require a hydrocarbon phase to disperse the gas hydrate crystals, they typically have a water-cut limitation. In general, the water-cut should be below 50% because otherwise the slurry becomes too viscous to transport.
Accordingly, there is an ongoing need for compositions and methods that effectively prevent agglomeration of gas hydrates in oil and gas transportation and handling processes, particularly at water-cuts greater than 50%.